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ЗмістModality Receptor type Afferent nerve fiber type and conduction velocity
118 8 Sensory systems
8.4 The physiology of the eye and visual pathways
The organization of the retina
The lachrymal glands and tear fluid
The intraocular pressure is maintained by the
The blink reflex and the dazzle reflex
The ciliary muscles control the focusing of the eye
The visual field
Photopic and scotopic vision
Visual acuity varies across the retina
The role of the retina in visual processing
The neurons of the visual cortex may have complex visual fields
8.5 The physiology of the ear—hearing and balance
We smell the air, taste our food, feel the earth under our feet, hear and see what is around us. To do all this, and more, we must have some means of converting the physical and chemical properties of the environment into nerve impulses which are the common coinage of the nervous system. The process of transforming some property of the external (or internal) environment into nerve impulses is called sensory transduction. It is carried out by specialized structures called sensory receptors, often simply called receptors.
Sensory receptors may be classified in several ways, of which two will be considered here (Table 8.1). They may be classified on the basis of the specific environmental qualities to which they are sensitive—chemoreceptors, mechanoreceptors, nociceptors, photo-receptors, and thermoreceptors. Alternatively, they may be classified according to the source of the quality that they sense. Thus
there are receptors that sense events that originate at some distance from the body—the eye, ear, and nose—which are sometimes called teleceptors. Others sense changes occurring in the immediate external environment—touch, pressure, and temperature. These are called exteroceptors. Then there are receptors that signal changes in the internal environment—the interoceptors. These sense blood pressure (baroreceptors), the oxygen and carbon dioxide levels of the blood (chemoreceptors), as well as substances released as a result of tissue damage (nociceptors). Other receptors provide information about our position in space and the disposition of our limbs—the gravitational receptors and propnoceptors.
For a particular receptor there is usually one kind of stimulus to which it will be especially sensitive. This is known as the adequate stimulus. Thus vibration is the adequate stimulus for the Pacinian corpuscles of the skin, small changes in temperature excite specific cutaneous thermoreceptors, light is the adequate stimulus for the photoreceptors of the eye, and so on.
Table 8.1 Classification of sensory receptors
Mechanoreceptors Special senses (ear)
Muscle and joints Skin and viscera
Special senses Skin and viscera
Cochlear hair cells
The hair cells of the vestibular system
Muscle spindles Golgi tendon organs
Pacinian corpuscle Ruffini ending Meissner's corpuscle Bare nerve endings
Arterial baroreceptors (sense high pressures) Atrial volume receptors (low-pressure receptors)
Glomus cells (carotid body, sense arterial Po2)
Hypothaiamic osmoreceptors and glucose receptors
Retinal rods and cones
Warm and cold receptors Temperature-sensing hypothaiamic neurons
Receptors send their information to the CNS (the brain and spinal cord) via afferent nerve fibers, often called primary afferents. A given afferent nerve fiber often serves a number of receptors of the same kind and will respond to a stimulus over a certain area of space and intensity that is called its receptive field. Receptive fields of neighboring afferents may, and often do, overlap, as shown in Fig. 8.1.
Individual nerve cells in the CNS may receive inputs from many primary afferent fibers, so that the receptive field of a particular sensory neuron in the spinal cord is usually larger than that of the afferents to which it is connected. This is called convergence. The second-order sensory nerve cells then make contact with many other nerve cells as information is processed by the CNS. Thus a specific piece of sensory information tends to be spread amongst more and more nerve cells. This is called divergence. The convergence and divergence of sensory information is an essential part of the processing of sensory information, which culminates in an internal representation of the world which the CNS can use to determine appropriate patterns of behavior.
single stimulus, nerve cells in the CNS receive inhibitory connections from neighboring cells via small neurons known as interneurons, as shown in Fig. 8.1. In this way a strongly excited cell will exert a powerful inhibitory effect on those of its neighbors that were less intensely excited. This is known as lateral inhibition, or surround inhibition, which features prominently in the processing of sensory information at all levels of the CNS. At the highest levels, in the cerebral cortex for example, this type of neural interaction allows the brain to extract information about the specific features of a stimulus. For example, the visual cortex is able to discriminate the position of an object in space, its illumination, and its relation to other nearby objects. All of this complex processing is achieved by the interplay of excitatory and inhibitory synaptic connections of the kind illustrated in Fig. 8.1. The synaptic basis of excitation and inhibition was described in Chapter 6.
Principles of transduction
Although different receptors respond to environmental stimuli in different ways, in all cases the adequate stimulus leads ultimately to a change in membrane potential, called a receptor
potential. For most sensory receptors, stimulation causes cation-permeable ion channels to open, which leads to the depolarization of the fiber and the generation of a receptor potential. When
the threshold is reached, an action potential is generated. The magnitude and duration of a receptor potential governs the number and frequency of action potentials transmitted by the afferent nerve fibers to the CNS. The basic steps in sensory transduction are illustrated in Fig. 8.2.
Receptors may either be bare nerve endings or may consist of specialized cells in close association with a nerve fiber. Bare nerve endings are commonly found in the skin and respond to touch and temperature. Other nerve endings respond directly to chemicals released into their local environment. For example, the sensory endings of pain fibers can be excited by a variety of substances, including hydrogen ions.
In the skin, encapsulated receptors are mechanoreceptors, examples are Merkel's disks and Pacinian corpuscles (Fig. 8.3). The intricate structure of these receptors permits the nervous system to discriminate the specific features of a stimulus. The role of the encapsulating cells in determining the characteristics of a particular kind of receptor is illustrated by the Pacinian corpuscle, which is surrounded by many layers of flattened fibro-blasts. An intact Pacinian corpuscle responds to deformation by a brief depolarization when the stimulus is first applied and when it is removed ('on' and 'off responses) as shown in Fig. 8.4. If the associated fibroblasts are removed following treatment with enzymes, the naked nerve ending remains depolarized for as long as rhe mechanical stimulus is applied. Thus, the fibroblasts allow the intact corpuscle to respond to rapid tissue movement (such as vibration) rather than maintained pressure.
The coding of stimulus intensity and duration
The nervous system needs to establish the location, physical nature, and intensity of all kinds of stimuli. Since most receptors
8 Sensory systems
Fig. 8.4 The role of the fibroblast lamellae in shaping the response of the Pacinian corpuscle to a pressure stimulus. The experimental arrangement is shown on the left and the receptor potentials are shown on the right. Note that the intact corpuscle signals the onset and offset of the stimulus while the receptor potential is maintained for the duration of the stimulus in the desheathed corpuscle.
Fig. 8.5 The response of a Pacinian corpuscle, rapidly adapting mechanoreceptor, and a slowly adapting mechanoreceptor to pressure stimuli applied to the skin. Each vertical spike represents an action potential. Note that the Pacinian corpuscle responds to maintained skin indentation with a single action potential while the slowly adapting mechanoreceptor continues to generate actin potentials. Only the Pacinian corpuscle is able to respond to the 300 Hz vibration of the skin.
respond to very specific stimuli, the primary afferent fibers to which they are connected can be considered as 'labeled lines'. For example, the skin has receptors that respond selectively to touch and others that respond to changes in temperature. The activation of a specific population of receptors will therefore inform the CNS of the nature and location of the stimulus. The intensity of the stimulus is coded both by the number of active receptors and by the number of action potentials it elicits. The timing and duration of the sequence of action potentials signals its onset and duration.
Many receptors generate action potentials when they are first stimulated but the action potential frequency falls with time even though the intensity of the stimulus is unchanged. This property is known as adaptation. Some receptors respond to the onset of a stimulus with a few action potentials and then become quiescent. This type of receptor is called a rapidly adapting receptor. Other receptors maintain a steady flow of action potentials for as long as the stimulus is maintained. These are known as slowly adapting or nonadapting receptors. These different types of response are illustrated in Fig. 8.5.
8.2 The somatosensory system
The skin is the interface between the body and the outside world. It is richly endowed with receptors that sense pressure,
8.2 The somatosensory system
touch, temperature, vibration, and pain. The muscles and joints also possess sensory receptors that provide information concerning the disposition and movement of the limbs (see pp 151—153). All of this information is relayed by the afferent nerves of the somatosensory system to the brain and spinal cord. Careful exploration of the skin has shown that it is not uniformly sensitive to touch, temperature, and pain. Specific points are sensitive to touch, while others are sensitive to cooling or warming. There is little overlap between the different modalities of cutaneous sensation.
The various kinds of receptors present in the skin are illustrated in Fig. 8.3. Bare nerve endings and encapsulated receptors are present. Each of the specific kinds of receptor subserves a specific submodality of cutaneous sensation. By identifying a point sensitive to touch, for example, and then excising it and subjecting it to histological examination it has been possible to associate particular types of receptor with specific modalities of sensation. Each kind of receptor is innervated by a particular type of nerve fiber. Pacinian corpuscles and Merkel's disks are innervated by relatively large A/3 myelinated nerve fibers, while the bare nerve endings that subserve temperature and pain are derived either from small AS myelinated fibers or from slowly conducting unmyelinated C-fibers (Table 8.2). (For the classification of nerve fibers see Table 6.2.)
In general, the receptive fields of touch receptors overlap considerably, as illustrated in Fig. 8.1 . The finer the discrimination
required, the higher the density of receptors, the smaller their receptive fields, and the greater the degree of overlap. The receptive fields for touch are particularly small at the tips of the fingers and tongue (about 1 mm2) where fine tactile discrimination is required. In other areas, such as the small of the back, the buttocks, and the calf, the receptive fields are about 100 times larger.
The distance between two points on the skin that can just be detected as separate stimuli is closely allied to the density of touch receptors and the size of their receptive fields. This is known as the two-point discrimination threshold. Not surprisingly, the greatest discrimination is at the tips of the fingers, the tip of the tongue, and the lips. It is least precise for the skin of the back (Fig. 8.6). A loss of precision in two-point discrimination can be used to localize specific neurological lesions.
The sensitivity of the skin to touch can be assessed by measuring the smallest indentation that can be detected, either subjectively or by recording the action potentials in a single afferent fiber. For the fingertips, an indentation of as little as 6—7 ^m can be detected, which corresponds to the diameter of a single red cell. Elsewhere on the hand the skin is less sensitive to deformation. For example, an indentation of about 20 /xm is required to evoke an action potential in an afferent fiber serving the sense of touch on the palm. The skin of the back or that of the soles of the feet is even less sensitive to touch.
Table 8.2 The receptor types and modalities of the somatosensory system
8 Sensory systems
Fig. 8.6 The variation of two-point discrimination across the body surface. Each vertical line represents the minimum distance that two points can be distinguished as being separate when they are stimulated simultaneously. Note the fine discrimination achieved by the fingertips, lips, and tongue, and compare this to the poor discrimination on the thigh, chest, and neck.
The skin has two kinds of thermoreceptor. One type responds specifically to cooling of the skin and another responds to warming. Their receptive fields are small—about 1 mm2 and they do not overlap. Exploration of the skin with a small temperature probe reveals specific points sensitive to cold or warmth. Histological examination of a cold or warm sensitive point shows only bare nerve endings. This suggests that the cutaneous thermoreceptors are probably a specific subset of bare nerve endings. The cold receptors are innervated by AS mye-linated afferents while the warm receptors are innervated by C-fibers.
Cutaneous thermoreceptors are generally insensitive to mechanical and chemical stimuli and maintain a constant rare of discharge for a particular skin temperature. They respond to a change in temperature with an increase or decrease in firing rate, the cold receptors showing a maximal rate of discharge around 25—30 °C while the warm receptors have a maximal rate of discharge around 40 °C (Fig. 8.7). Thus a given frequency of discharge from the cold receptors may reflect a temperature that is either above or below the maximum firing rate for a particular receptor. This ambiguity may explain the well known paradoxical sense of cooling when cold hands are being rapidly warmed by immersion in hot water.
Kinesthesia and haptic touch
People move through their environment, they lift objects, move them around, and feel their texture. The active exploration of an
Fig. 8.7 Typical response patterns of a warm and a cold skin thermoreceptor. (a) The action potential discharge of a cold receptor is seen to increase as the skin is cooled from 28 to 23 °C. (b) The rate of action potential discharge as a function of temperature for a single cold and a single warm thermoreceptor.
object to determine its shape and texture is known as haptic touch. It is clearly able to provide the brain with more information about an object than a single contact and is used to great effect by the blind.
The intrinsic knowledge of the position of the limbs even when blindfold is called kinestbesia. Two sources of information provide the brain with information about the limbs. These are the corollary discharge of the motor efferents to the sensory cortex (Chapter 9), which provides information about the intended movement, and sensory feedback which directly informs the sensory cortex of the actual progress of the movement. These two sources of information are required as the load on the muscles moving the limbs cannot be known by the brain in advance (see Chapter 9 for further information).
The importance of the muscle spindles in kinesthesia is revealed by the vibration illusion. A normal subject can accurately replicate the position of one arm by moving the other to the same angle. The application of vibration to a muscle such as the biceps can give the illusion that the arm has been moved. If both arms are initially placed at 60° to the horizontal and the left biceps is then vibrated, a blindfolded subject will have the illusion that the arm has moved and will alter the position of the right arm to report the perceived (bur incorrect) position of the left arm (Fig. 8.8). This occurs because the vibration stimulates the stretch receptors. The CNS interprets the increased afferent discharge as indicating that the muscle is longer than it actually is.
8.2 The somatosensory system
Fig. 8.8 An illustration of a limb position mismatch induced by vibratory stimulation. The blindfolded subject was asked to align her forearms while her left biceps was being vibrated. The mismatch illustrates the illusion in limb position sense elicited by the vibration.
The internal organs are much less well innervated than the skin. Nevertheless, all of the internal organs have an afferent innerva-tion, although the activity of these afferents rarely reaches consciousness except as a vague sense of 'fullness' or as pain. The afferent fibers reach the spinal cord by way of the visceral nerves which also carry the sympathetic and parasympathetic fibers that provide motor innervation to the viscera (Chapter 10).
The visceral receptors include both rapidly adapting and slowly adapting mechanoreceptors and chemoreceptors. Many of these afferents are an essential component of visceral reflexes that control vital body functions. Examples are the baroreceptors of the aortic arch and carotid sinus which monitor arterial blood pressure, and the chemoreceptors of the carotid bodies that detect the Po2, Pco2, and pH of the arterial blood and play an important role in the regulation of breathing (Chapter 16).
Afferent information from the
somatosensory receptors reaches the
brain via the dorsal columns and the
The cutaneous and visceral afferent fibers enter the spinal cord via the dorsal (posterior) roots. The large-diameter afferents branch after they have entered the spinal cord and travel in the dorsal columns to synapse in the dorsal-column nuclei (the cuneate and gracile nuclei) of the medulla oblongata. The second-order fibers leave the dorsal-column nuclei as a discrete fiber bundle called the medial lemniscus. The fibers first run anteriorly and then cross the mid-line before reaching the ventral thalamus. From rhe thalamus they project to the somatosensory regions of the cerebral cortex (Fig. 8.9).
The small afferent fibers join a bundle of fibers at the dorso-lateral margin of the spinal cord known as Lissauer's tract. These thin afferent fibers only travel for a few spinal segments at most before entering the gray matter of the spinal cord, where they
Fig. 8.9 The dorsal-column lemniscal pathway for cutaneous sensation.
synapse on spinal interneurons. These interneurons then synapse on the neurons whose axons form the spinothalamic tracr. The axons from these neurons then cross the mid-line and project to the thalamus via the spinothalamic tract which runs in the anterolateral quadrant of the spinal cord. From the thalamus, the sensory projections reach the somatosensory regions of cerebral cortex as shown in Fig. 8.10. Finally, mention must be made of the spinorericular tract which receives afferents from the sensory nerves and projects mainly to the reticular formation of the brainstem on the same side. These fibers ascend in the anterolateral spinal cord. This tract consists of a chain of short fibers which synapse many times as they ascend the spinal cord.
Since the dorsal column contains fibers from the largest of the afferent fibers, which are mainly concerned with touch and pro-prioception, this fiber tract sends information to the brain that is concerned with fine discriminatory touch, vibration, and position sense (kinesthetic information). The spinothalamic and spinoreticular tracts receive information from the smaller afferent fibers and so transmit information concerning crude touch, temperature, and pain to the brain (Table 8.2).
All of the afferents of a particular type that enter one dorsal root tend to run together in the lower regions of the spinal cord.
8 Sensory systems
Fig. 8.10 The spinothalamic tract and its projection to the cerebral cortex.
Initially this segmental organization is preserved but, as they ascend, the fibers from the different segments become rearranged so that those from the leg run together, as do those of the trunk, hand, and so on. Thus, for example, the afferents from the hand project to cells in a particular part of the dorsal column nuclei and thalamus, while those from the forearm project to an adjacent group of cells. This orderly arrangement provides a topographical map of the body.
Although the cells of the dorsal column nuclei project chiefly to the specific sensory nuclei in the ventrobasal region of the thalamus, those from the spinothalamic tract have a wider distribution and also project to the intralaminar nuclei and the posterior thalamus. The sensory cells of the thalamus project to two specific regions of the cerebral cortex.
The exploration of the human somatosensory cortex by Penfield and his colleagues was one of the most remarkable investigations in neurology. While treating patients for epilepsy, Penfield attempted to localize the site of the lesions by stimulating the cortex electrically while his patients were conscious but under local anesthesia. Since the brain has no nockeptive fibers, this procedure did not elicit pain but it did elicit specific move-
Fig. 8.11 The representation of the body surface on the post-central gyrus revealed by electrical stimulation of the cerebral cortex of conscious subjects.
ments or specific sensations, depending on which area of the cortex was stimulated. Systematic exploration of the post-central gyrus revealed that it was organized somatotopically, with the different regions of the opposite side of the body represented as shown in Fig. 8.11. The genitalia and feet are mapped onto an area adjacent to the central fissure, while the face, rongue, and lips are mapped on the lateral aspect of the post-central gyrus. Although the area of representation is disproportionate with respect to the body surface, it is appropriate to the degree of importance of the different areas in sensation. Thus the hands, lips, and tongue all have a relatively large area of cortex devoted to them compared to the areas devoted to the legs, upper arm, and back. The body is also mapped on to the superior wall of the sylvian gyrus. This area is called rhe SII region. Unlike rhe post-central gyrus (the SI region), the SII region has a representation of both sides of the body surface.
The trigeminal system
The sensory inputs from the face are relayed to the brain via the fifth cranial nerve, the trigeminal nerve. The trigeminal nerve is mixed, having both somatic afferent and efferent fibers, although the afferent innervation dominates. It arises in the pontine region of the brainstem and, shortly after its origin, it expands to form the semilunar ganglion which contains the primary sensory neurons, which are analogous to the dorsal root ganglion
neurons. Three large nerves leave the ganglion to innervate the face. These are the ophthalmic, maxillary, and mandibular nerves. These nerves relay information to the brain concerning touch, temperature, and pain from the face, the mucous membranes, and the teeth. The large afferent fibers transmit information from the mechanoreceptors to the thalamus via the medial lemniscus (see Fig. 8.9) while the AS- and C-fiber afferents (which are mainly concerned with temperature and nociception) join the spinothalamic tract as shown in Fig. 8.10. From the thalamus, information from both large and small afferents is transmitted to the face region of the primary sensory cortex.
Pain is the sensation we experience when we injure ourselves or when we have certain organic diseases. It is an unpleasant experience, which we associate with tissue damage. The weight of evidence now suggests that pain is conveyed by specific sets of afferent nerve fibers and is not simply the result of massive stimulation of afferent fibers generally. Nevertheless, pain may arise spontaneously without an obvious organic cause, or in response to an earlier injury, long since healed. This kind of pain often has its origin within the CNS itself. Although it is not obviously associated with tissue damage, pain of central origin is no less real to the patient. Unlike most other sensory modalities, pain is almost invariably accompanied by an emotional reaction of some kind, such as fear or anxiety. If it is intense, pam elicits autonomic responses such as sweating and an increase in blood pressure and heart rate.
Pain may be classed under one of three headings:
1. There is pricking pain which is rapidly appreciated, accurately localized, and which elicits little by way of auto-
nomic responses. This kind of pain is usually transient and has a sharp pricking quality. It is sometimes called 'first' or 'fast' pain and is transmitted to the CNS via small mye-linated A<5-fibers. In normal physiology it serves an important protective function as activation of these fibers triggers the reflex withdrawal of the affected region of the body from the source of injury.
Nociceptors are activated by specific substances released from damaged tissue
As for the sense of touch and temperature, the distribution of the pain receptors in the skin is punctate. Histological examination of a pain spot reveals a dense innervation with bare nerve endings which are believed to be the nociceptors. The adequate stimulus for the nociceptors is not known with certainty but the application of pain-provoking stimuli such as radiant heat elicits reddening of the skin and other inflammatory changes. This suggests that one stimulus for pain is the rate of destruction of tissue that is innervated by pain fibers. It is probable that a number of chemical agents, called pain-producing substances, are released following injury to the skin and cause the pain endings to discharge. These include ATP, bradykinin, histamine, serotonin (5HT), hydrogen ions, and a number of inflammatory mediators, such as prostaglandins.
The triple response
If a small area of skin is injured, for example by a burn, there is a local vasodilatation which elicits a reddening of the skin. This is followed by a swelling (a weal or wheal) which is localized to the site of the injury and its immediate surroundings. The original site of injury is then surrounded by a much wider area of less intense vasodilatation known as the 'flare'. The local
Fig. 8.12 The axon reflex that gives rise to the flare of the triple response. Nociceptive fibers from the skin branch and send collateral fibers to nearby blood vessels. If the skin is injured, action potentials pass to the spinal cord, via the dorsal root, and to the axon collaterals, where they cause a vasodilatation which gives rise to the flare. If the nerve trunk is cut at point 'X' and sufficient time allowed for the nerve to degenerate, the flare reaction is abolished. If the cut is made beyond the dorsal root ganglion at point 'Y', the segmental spinal reflexes are abolished but not the flare reaction.
reddening ('red reaction'), flare, and weal formation comprise the triple response that was first described by Thomas Lewis. The weal is a local edema due to the accumulation of fluid in the damaged area. The red reaction is due to arteriolar dilatation in response to vasodilator substances released from the damaged skin, while the flare is due to dilatation of arterioles in the area surrounding the site of injury.
Within the injured area and across the surrounding weal, the sensitivity to mildly painful stimuli such as a pinprick is much greater than before the injury. This is known as primary hyper-algesia, which may persist for many days. In the region covered by the flare, outside the area of tissue damage, there is also an increased sensitivity to pain which may last for some hours. This is known as secondary hyperalgesia.
Unlike the weal and local red reaction, the flare is abolished by infiltration of the skin with local anesthetic. It is not blocked, however, if the nerve trunk supplying the affected region is anesthetized. Moreover, if the nerve trunk is cut (e.g. at point X in Fig. 8.12) and allowed to degenerate before eliciting the triple response, only the local reddening and weal formation occur— the flare is absent. The triple response can be elicited in animals that have had complete removal of their sympathetic innerva-tion. Furthermore, cutting the nerve supply to the dorsal root of the appropriate segment (point Y of Fig. 8.12) does not block the flare reaction. These experiments show that the flare is a local axon reflex rather than a reflex vasodilatation involving the brain-stem.
CNS pathways in pain perception
Nociceptive fibers are a specific set of small-diameter dorsal root afferents that subserve pain sensation in a particular region.
These fibers enter Lissauer's tract and synapse close to their site of entry into the spinal cord. The second-order fibers cross the mid-line and ascend to the brainstem reticular formation and thalamus via the spinothalamic tract and spinoreticular tracts. The fibers of the spinoreticular tract may be concerned with cortical arousal mechanisms and eliciting the defense reaction (Chapter 15).
Although the neurons of the ventrobasal thalamus project to the primary sensory cortex, there is no compelling evidence that noxious stimuli evoke neural responses in this region. Electrical stimulation of the posterior thalamic nuclei in conscious human subjects does, however, elicit pain. Moreover, degenerative lesions of the posterior thalamic nuclei can give rise to a severe and intractable pain of central origin known as thalamic pain. It is known that the neurons of the posterior thalamus project to the secondary sensory cortex on the upper wall of the lateral fissure, and electrical stimulation of the white matter immediately beneath the secondary somatosensory cortex (SII) also elicits the sense of pain. This suggests that the SII region is concerned with the perception of pain. Other areas that appear to be involved in the whole pain experience are the reticular formation, the structures of the limbic system such as the amygdala, and the frontal cortex.
Fig. 8.13 The descending pathways that inhibit the activity of nociceptive neurons in the dorsal horn. The different sections are not drawn to scale.
The perception of pain may be greatly modified by circumstances. It is widely known that the pain from a bruise can be relieved by vigorous rubbing of the skin in the affected area. Pain can also be relieved by the electrical stimulation of the peripheral nerves. In both cases the large-diameter afferent fibers
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